1. Field of the Invention
The present invention relates generally to optical waveguide fiber having reduced polarization mode dispersion (PMD), and particularly to optical waveguide fiber having reduced PMD due to an impressed pattern of change in fast axis of birefringence.
2. Technical Background
Polarization Mode Dispersion becomes a significant source of inter-symbol interference in high data rate systems, especially those systems designed for longer spacing between opto-electronic signal regenerators. As a result, optical waveguide fibers having reduced PMD have been developed, as typified in U.S. Pat. No. 5,943,466, Henderson et al. incorporated herein by reference in its entirety. In the Henderson patent is disclosed an optical waveguide fiber and a method of making the fiber in which the fast axis of birefringence (and thus the orthogonal slow axis of birefringence) changes in direction in accord with a pattern impressed on the fiber. The pattern can be impressed by spinning the fiber, in accord with a spin function, during the drawing process. As is described in the Henderson patent, the spin function instructs a spinning apparatus to effect a pre-selected pattern of relative rotation (relative spin) between the preform and the fiber being drawn therefrom. By relative rotation is meant either the fiber or the preform or both of these are spun to produce the desired change of birefringence axis. Spin functions set forth in the Henderson patent include frequency and amplitude modulated sinusoidal functions.
Because PMD is effected by fiber properties, such as beat length, as well as by external factors, such as cabling method and materials, work has continued to find spin functions which are sufficiently simple in form to allow ease of implementation in a manufacturing environment. At the same time, the spin functions must be robust in the sense that they produce the desired reduction in PMD under normally encountered manufacturing variations and under a variety of cabling and installation conditions.
Typical, cost effective, spinning apparatus will impress a spin pattern on the optical waveguide fiber that deviates somewhat from the pattern prescribed by the spin function. For example, the spinning apparatus may not be sufficiently responsive to follow the pattern defined by the spin function. There is a need, therefore, for identification of spin functions which in addition to reducing PMD for a wide range of beat lengths also maintain the PMD reduction under conditions of normally encountered manufacturing variations.
The present invention addresses the problem of designing a family of spin functions that produce a selected reduction in PMD despite relatively large deviations of actual impressed spin pattern from target spin pattern, the deviations being due to limitations inherent in the spinning apparatus or to variations in draw speed.